Wavelength division multiplexing (WDM) is a technique for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical channels are carried over a single waveguide, each channel being assigned a particular wavelength. Through the use of optical amplifiers, such as rare earth doped fiber amplifiers, these optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical networks. Dense WDM or DWDM systems are also employed which have a greater number of optical channels with smaller channel spacings.
The transmitters used in WDM systems typically include semiconductor lasers, for example Distributed Feedback lasers (DFB), where each laser transmits light at a designated one of a plurality of wavelengths. DFB lasers generally comprise one or more III-V semiconductor materials and are commercially available. Each laser outputs an optical carrier signal at a particular channel wavelength usually within the 1.55 .mu.m range which corresponds to an absorption minimum associated with silica-based fibers.
When a laser transmitter used in a WDM system is first turned on, it experiences a "ramp-up period" where the drive current must first increase to a level where the semiconductor laser provides light at the desired wavelength and at operating power. During this ramp-up period, the laser transmitter is not yet emitting light at the desired wavelength, however light still propagates down the transmission fiber. In addition, during transmitter operation, the laser output may drift off-channel allowing unwanted light to propagate down the fiber. In either case, this unwanted light transmission may adversely impact adjacent channel performance and compromise the integrity of the transmitted information signals.
To control the output power and frequency of each laser in a WDM system, a laser controller is used to provide the required laser bias current as well as thermal control of the laser. By using thermal control which can be responsive to feedback information from a wavelength reference, the temperature of the laser diode can be adjusted so that the reference signal remains stable or at a peak, thereby maintaining the precise operating frequency of the laser.
In order to achieve high bit-rate transmission over long distances, the DFB laser sources used in DWDM communication systems are modulated externally with an information signal. This is because externally modulated DFB lasers have relatively low wavelength chirp as compared with laser diodes being directly modulated. An exemplary external modulator, such as a Mach-Zehnder modulator, employs a waveguide medium, whose refractive index changes according to the applied electrical field, i.e., the refractive index of an electro-optic material such as LiNbO.sub.3 can be changed by applying an external voltage. In a Mach-Zehnder configuration, two optical interferometer paths are provided where an incoming optical carrier is split between the two paths. In the absence of an external voltage, the optical fields in the two paths constructively interfere. When an external voltage is applied, the refractive index in the waveguides is altered which induces a phase shift that produces destructive interference at its output.
When a LiNbO.sub.3 modulator is employed with a semiconductor light source in a WDM system, it would be advantageous to turn the modulator off in order to prevent the transmission of unwanted light from propagating down the optical fiber. Once the drive current would increase to a level where the laser provides light at the desired channel wavelength at the operating power, the modulator may be modulated to produce the desired information signals. However, LiNbO.sub.3 modulators without attenuators retain an unwanted electrostatic charge in the off-state which distorts the output signal which may then propagate down the transmission fiber and cause bit errors at the receiving end.
This can be seen with reference to FIG. 1 which represents a digital bit stream of ones and zeros modulated by a typical LiNbO.sub.3 modulator. A true "on" state representing a one bit corresponds to a voltage level of V.sub.ON and a true "off" state representing a zero bit corresponds to a voltage level of V.sub.OFF. As stated above, a LiNbO.sub.3 modulator retains an unwanted electrostatic charge such that the voltage level in the "off" state corresponds to V.sub.0. Similarly, when the LiNbO.sub.3 modulator is in the "on" state, the voltage level corresponds to V.sub.1. In other words, this retained charge produces a voltage level which distorts the output signal representing a one or a zero in the digital bit stream which may cause bit errors at the receiving end, thereby compromising signal integrity.
Thus, there is a need to provide a simple and cost effective optical device which provides a crosstalk-free signal source for multiple wavelength optical transmission systems.